DESCRIPTION: This proposal will test the hypothesis that a postsynaptic neuron can determine the form of synaptic plasticity exhibited by the presynaptic terminals that contact it. Previous studies implicating the postsynaptic neuron as a mediator of presynaptic plasticity have relied on correlative evidence. However, in the cricket Acheta domesticus it is possible to directly test this hypothesis by inducing the formation of 'novel' synapses; synapses between pre- and postsynaptic partners that would normally never form. For example, when presynaptic neurons that exhibit synaptic depression are transplanted to ectopic locations they synapse with a postsynaptic neuron whose normal synaptic connections always express facilitation. Preliminary experiments have revealed that the form of short-term plasticity exhibited by these ectopic neurons is influenced by the new postsynaptic target with which they synapse. The large, identifiable nerve cells of the cricket allow the synaptic connections in this system to be readily accessible to intracellular analysis. Electrophysiological techniques will be used to describe the short-term plasticity expressed at both control and novel synapses. The primary aim of these experiments will be to investigate the physiological changes associated with the formation of these novel synaptic connections. In addition, an ultrastructural analysis will first determine if synapses expressing different forms of plasticity are structurally distinct in this system and then establish if the change in plasticity observed at a novel synapse is accompanied by a change in synaptic structure. Activity-dependent changes in both the physiology and morphology of central synaptic connections play an important role in nervous system development, behavioral plasticity and in the neural processing involved in learning and memory acquisition. The results of the experiments described in this proposal will provide the foundation for further investigation into the mechanisms regulating this synaptic plasticity.